Schematic view of the dominant H₂ formation routes depending on the density, the dust fraction, and visible radiation field W summarizing our results presented in Sect. 3.1 and in Appendix C. The location of the boundaries are given for a temperature of T_K = 1000 K, x_e = 4.8 × 10⁻⁴, and x(C⁺) = 3.6 × 10⁻⁴. The schematic view remains valid from ≃100 K up to ≃ 5000 K. Dependencies on T_K, x_e, and x(C⁺) are given inAppendix C. We note that some limits depend on the visible flux W and others do not. Depending on the visible flux, boundaries ①, ②, and ③ can merge.